The present invention relates to a heating element made of carbon, which is installed under road surfaces to be used for melting snow or under floors for heating purposes, the heating element comprising a heating member formed of carbon powder and an insulating resin kneaded together.
The heating element of this type consumes less electricity than the known Nichrome wire, and the heating member per se has a temperature control function with the insulating resin repeating expansion and contraction with temperature variations thereby controlling an electric current flow. Therefore, in recent years this heating element is employed in floor heating and various other applications, and is marketed in planar and linear forms. However, the heating element commercially available heretofore has the disadvantage that the heating temperature greatly varies at locations of the heating element, failing to assure uniform heating.
The present inventor has conducted various tests in search of the cause of such a drawback, and has found the cause in the construction of the heating element. The conventional heating element, taking one in linear form as shown in FIG. 5 of the accompanying drawings for example, comprises a solid heating member 2' having an about 4 mm diameter and peripherally coated with an insulating member 4' having an about 1 mm thickness. Such heating element 1' usually is formed by extruding the heating member 2' and insulating member 4' together from an extruding machine and immediately cooling the same by cooling water or other means. It is therefore inevitable that the cooling progresses by degrees from surface to inside, which causes the composition of the heating member 2' to be nonuniform in the radial direction and even creates numerous voids A in the center. It has been found that, because of the non-uniformity in the composition of the heating member 2' and the presence of voids A, the electric resistance of the heating member 2' greatly varies from one location thereof to another, which results in non-uniformity in its heating temperature. Even if a thermister, for example, is incorporated from the safety point of view to control the temperature, the uneven heating temperature results in certain locations becoming very hot. This renders the use of the thermister meaningless and gives rise to a safety problem. Such a phenomenon occurs with the planar heating element also. Furthermore, in the case of linear heating element 1', expansion of the insulating member occurring with the heating greatly elongates the heating element 1'. Where a plurality of heating elements 1' are jaxtaposed, adjacent heating elements when elongated tend to contact each other causing a short circuit.